As resources are becoming scarcer, great efforts are currently being made to increase the proportion of renewable energy sources in the overall energy supply in comparison with the proportion of conventional energy sources. Conventional energy sources, for example gas, coal-fired or nuclear power plants, are characterized by a consumption of non-regenerative raw materials, whereas renewable energy sources are based on energy carriers which regenerate.
However, in comparison with conventional energy sources, renewable energy sources are often subject to fluctuations in the currently available energy carriers, for example the instantaneous solar radiation in photovoltaic installations or the instantaneous wind strength in wind power plants, as a result of which the power which can be supplied to a common power supply network may likewise vary greatly. Since a multiplicity of energy sources and consumers are often connected to a typical power supply network, a certain amount of coordination of the various suppliers is required. In an AC voltage network, the network voltage and the network frequency, in particular, may have only small ranges of variation. These supply parameters are usually controlled in such a manner that individual fluctuations in the various energy sources are at least partially compensated for by means of negative feedback effects. For example, a surplus of electrical power in a conventional energy source results in its operating frequency, that is to say the frequency of the AC voltage supplied to the AC voltage network by this energy source, increasing. This results in more power flowing into the AC voltage network on account of the positive phase shift of the supplied AC voltage, thus increasing the individual load on this energy source and thus counteracting a further increase in the operating frequency.
The renewable and conventional energy sources are currently usually coordinated by virtue of the renewable energy sources being connected to the AC voltage network as current sources and the conventional energy sources being connected to the AC voltage network as voltage sources. In this manner, a renewable energy source can vary the supplied current according to the currently available power while automatically adapting to the voltage currently present in the AC voltage network. In contrast, the conventional energy sources operated as voltage sources are responsible for forming the network, that is to say for specifying operating parameters of the AC voltage network, for example the network voltage, and for providing the power required for this purpose.
In the type of coordination described above, the renewable energy sources are operated at the maximum power point (or MPP), however, only as long as the network frequency remains below a predefined threshold value. Above the threshold value, the available power from the renewable energy sources is generally not completely used at the expense of an increased resource consumption of the conventional energy sources.